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Article

Enhancing the Performance of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) Perovskites as Catalysts for CO Oxidation by Decreasing the Ba Content

by
Á. Díaz-Verde
and
M. J. Illán-Gómez
*
Carbon Materials and Environment Research Group, Inorganic Chemistry Department, University of Alicante, Ctra San Vicente del Raspeig s/n, San Vicente del Raspeig, 03690 Alicante, Spain
*
Author to whom correspondence should be addressed.
Nanomaterials 2024, 14(16), 1334; https://doi.org/10.3390/nano14161334
Submission received: 10 July 2024 / Revised: 29 July 2024 / Accepted: 7 August 2024 / Published: 10 August 2024
(This article belongs to the Special Issue Synthesis and Applications of Perovskite Nanocrystals)
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Abstract

:
Mixed oxides featuring perovskite-type structures (ABO3) offer promising catalytic properties for applications focused on the control of atmospheric pollution. In this work, a series of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) samples have been synthesized, characterized and tested as catalysts for CO oxidation reaction in conditions close to that found in the exhausts of last-generation automotive internal combustion engines. All samples were observed to be active as catalysts for CO oxidation during CO-TPRe tests, with Ba0.7MnO3 (B0.7M) being the most active one, as it presents the highest amount of oxygen vacancies (which act as active sites for CO oxidation) and Mn (IV), which features the highest levels of reducibility and the best redox properties. B0.7M has also showcased a high stability during reactions at 300 °C, even though a slightly lower CO conversion is achieved during the second consecutive reaction cycle. This performance appears to be related to the decrease in the Mn (IV)/Mn (III) ratio.

1. Introduction

Currently, the scientific community is focused on looking for solutions to minimize the emissions of polluting gasses, which come from, among other sources, the industrial and automotive sectors. In addition to CO2, which is one of the most problematic species in the automobile sector [1,2], CO is considered to be an anthropogenic toxic pollutant if its concentration exceeds the amount established by environmental regulations [3]. The catalyzed CO oxidation is the main strategy for the removal of CO from the automobile exhaust, which is one of the main current sources. Pt-based formulations are among the most effective catalysts [4,5,6] for CO oxidation. However, the use of noble metal-based catalysts presents two main drawbacks: (i) the high cost, due to their limited availability [7]; and (ii) the sintering at high temperatures [8,9], which causes a remarkable decline in the catalytic performance. Consequently, the development of affordable catalysts that could match the catalytic performance of noble metals but avoid the sintering problem is an urgent target. In recent years, several transition metal oxides have been identified as potential catalysts for CO oxidation, such as Co3O4, CuO, MnO2 and CeO2 [10,11,12,13].
In this sense, transition metal-based mixed oxides with perovskite structures (ABO3) are considered to be an interesting family of solids for, among other applications [14,15], oxidation reactions and, thus, also for catalytic CO oxidation [16]. This is because their structure allows for the stability of a huge amount of non-noble metal mixed oxides with the appropriate cationic radii to fit well in the A (12 coordination) and B (6 coordination) sites. Specifically, A-sites are usually occupied by rare earth, alkaline and alkaline–earth metals; meanwhile, transition metals commonly occupy the B-sites [17]. This structure also enables the partial substitution of A and/or B cations, leading to the creation of structural defects and a variable oxygen non-stoichiometry. These characteristics, as well as the high stability and redox properties, allow perovskite oxides to present a wide applicability in catalytic reactions. The redox processes of these solids, both on the surface and in the bulk, account for the reversible loss and uptake of oxygen and the creation and filling up of oxygen vacancies [16,18,19]. In fact, manganese-based perovskites have been proposed as convenient catalysts for CO oxidation [20], as this metal presents not only atomic orbitals with the appropriate symmetry and energy levels for the activation of CO and O2 molecules [21] but also a great versatility for modifying its oxidation state, thus favoring the redox processes [22,23]. In perovskites, the electronic configuration of B cation plays a crucial role in the catalytic performance for oxidation reactions, as it determines the adsorption and the activation of the reactants (CO and O2 in the context of CO oxidation). In fact, the occupancy of the eg orbital, which presents the optimal geometry for interacting with the molecular orbitals of reactants, dictates the energies involved in the adsorption and the desorption of reactants [16], as the energy of adsorption increases as the number of electrons in the eg orbital decreases. For Mn (III), an effective interaction with CO molecules exits, since three electrons are located in the t2g orbitals and one is located in the eg orbitals, so the partially empty eg orbital accepts the pair of lone electrons from the carbon atom of CO while backdonation of a t2g electron to the antibonding π* orbital of the CO molecule occurs [21]. Conversely, Mn (IV) presents only three electrons in the t2g orbitals (and lacks electrons in eg orbitals), affecting its interaction with CO and O2 through the strength of the Mn-O bond. Thus, by adjusting the physicochemical properties of Mn-based perovskites, and hence for the Mn (IV)/Mn (III) ratio, it is possible to weaken the Mn-O bond, enhancing the catalytic activity for CO oxidation [24].
In a previous work developed by the authors [25], the advantages of employing a Ba-deficient perovskite formulation (Ba0.7MnO3) for the CO oxidation in simulated gasoline car exhaust conditions were established. In this paper, the purpose is to try to enhance the performance of Ba0.7MnO3 perovskite by determining the effect of the gradual decrease in Ba content in the catalytic activity of the BaMnO3 mixed oxide through the synthesis and characterization of a BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) series of perovskites to be used for CO oxidation in different conditions, simulating the composition of last-generation diesel and gasoline car exhausts. The detailed study of the gradual decrease in the Ba is of great importance, due to the possibility of finding optimal formulations with intermediate A cation contents, as reported in the literature [26].

2. Materials and Methods

The BaxMnO3 (BxM) perovskites were synthesized using the sol–gel method adapted to an aqueous medium [27,28]. Briefly, first, a 40 mL solution (containing EDTA (C10H16N2O8, Sigma–Aldrich, St. Louis, MI, USA, 98.5% wt) used as a chelating agent in a 1:2 molar Mn:EDTA ratio) at 60 °C was prepared. Barium nitrate (Ba(NO3)2, Sigma–Aldrich, 99.0% wt) and manganese(II) nitrate tetrahydrate (Mn(NO3)2·4 H2O, Sigma–Aldrich, 99.0% wt) were added as metal precursors and, afterward, citric acid (C6H8O7, Sigma–Aldrich, 99.0% wt) was incorporated using the same molar ratio than for EDTA (1:2, Mn:citric acid). Subsequently, the temperature was increased to 80 °C for assuring the gel formation. During all the procedure, the pH was set at 9 by employing an ammonia solution (NH3, Panreac, Castellar del Valles, Spain, 30% wt). Finally, the obtained gel was dried at 150 °C during 12 h and, as the last step, the solid was calcinated at 850 °C for 6 h.
For a sample characterization, the following techniques have been used.
The surface composition of samples was determined by μ-XRF, employing an Orbis Micro-XRF Analyzer from EDAX.
The textural properties were determined by N2 adsorption at −196 °C using an Autosorb-6B instrument from Quantachrome (Anton Paar Austria GmbH, Graz, Austria). The samples were degassed at 250 °C for 4 h before the N2 adsorption experiments.
The crystalline structure was obtained using X-ray Diffraction (XRD). The X-ray patterns were recorded between 20° and 80° 2θ angles with a step rate of 0.4 °min−1 and using Cu Kα (0.15418 nm) radiation in a Bruker D8-Advance device. Average crystallite sizes [29] and lattice strain values [30] were determined employing the Williamson–Hall method. Finally, XRD refinement was performed to determine the percentage of the different crystalline phases in the sample using the HighScore Plus software (Malvern Panalytical B.V. Almelo, The Netherlands, 4.9 (4.9.0.27512) version).
The surface chemistry was evaluated by X-ray Photoelectron Spectroscopy (XPS) using a K-Alpha Photoelectron Spectrometer by Thermo-Scientific with an Al Kα (1486.7 eV) radiation source. To obtain the XPS spectra, the pressure of the analysis chamber was maintained at 5 × 10−10 mbar. The binding energy (BE) and kinetic energy (KE) scales were adjusted by setting the C 1s transition at 284.6 eV, and the BE and KE values were then determined with the peak-fit software of the spectrometer (Thermo Avantage v5.9929). To obtain the collected data, the following transitions were analyzed: O 1s, Mn 2p3/2, Mn 3p and Ba 3d5/2.
The reducibility of samples was estimated by Temperature-Programmed Reduction with H2 (H2-TPR) in a Pulse Chemisorb 2705 (from Micromeritics) provided with a Thermal Conductivity Detector (TCD) and using 30 mg of sample, which was heated at 10 °C min−1 from 25 °C to 1000 °C in 5% H2/Ar atmosphere (40 mL min−1). The quantification of the H2 consumption was carried out employing a copper (II) oxide (CuO, Sigma–Aldrich, 99.9% wt) reference sample, which is known to be reduced according to reaction (1) [31]. For the assignment of the H2 consumption peaks detected in the H2-TPR profiles, manganese (III) oxide (Mn2O3, Sigma–Aldrich, 99% wt) and manganese (IV) oxide (MnO2, prepared using the same manganese precursor and calcination conditions as those used for the perovskites synthesis, were employed as references, being then reduced according to reactions (2) and (3), respectively [32].
CuO + H2 → Cu + H2O
Mn2O3 + H2 → 2MnO + H2O
MnO2 + H2 → MnO + H2O
O2-TPD experiments were performed in a Thermo Gravimetric Mass Spectrometrer (TG-MS) system (Q-600-TA and Thermostar from Balzers Instruments (Pfeiffer Vacuum GmbH, Aßlar, Germany) respectively), with 16 mg of sample heated at 10 °C min−1 from room temperature to 950 °C under a 100 mL/min of helium atmosphere. Before the experiments, each sample underwent a 1 h preheating treatment process at 150 °C to remove the moisture. During the experiments, the 32 m/z signal was followed for the O2 evolved during the experiments. The amount of evolved oxygen was also estimated using a CuO reference sample, which is decomposed to Cu2O under the tested conditions [33], according to reaction (4).
4CuO → 2Cu2O + O2
Temperature-Programmed Reduction with CO (CO-TPR) tests were carried out in order to obtain information about the conversion of CO into CO2 using exclusively the oxygen evolving from the perovskite samples. To obtain the CO-TPR profiles, the samples (50 mg of catalyst and 100 mg of SiC loaded into a U-shaped quartz reactor) were subjected to a heating at 10 °C min−1 until 600 °C in a 100 mL min−1 flow (Gas Hourly Space Velocity (GHSV) of 4967 h−1) of a 1% CO/He gas mixture [34]. Before starting the CO-TPR test, the mixture catalyst-SiC was preheated for 1 h at 700 °C in a 5% O2/He gas mixture for cleaning the surface of samples. Subsequently, an additional preheating (following the same conditions) was performed prior to the second CO-TPR cycle. For reaction product quantification, an Agilent 8860 gas chromatograph was used, provided with a TCD and two packed columns (Porapack-Q and MolSieve-13X (Agilent Technologies Spain, Madrid, Spain)). The CO conversion was calculated by employing Equation (1):
CO conversion (%) = ((COin − COout)/COin) · 100
where COout is the outlet molar flow rates of CO and COin is the inlet molar flow rate.
To determine the catalytic activity for CO oxidation, Temperature-Programmed Reaction (CO-TPRe) tests have been developed using three gas mixtures composed of the following: (i) 1% CO and 1% O2 in He, as an approximation to the CO partial pressure in the actual TWC working conditions [34]; (ii) 1% CO and 10% O2 in He, for analyzing the effect of using a higher oxygen partial pressure in relation to (i); and (iii) 0.1% CO and 1% O2 in He, for determining the effect on the catalytic performance in a very low CO partial pressure in relation to (i). For these experiments, the same reactor configuration and preheating treatment described for the CO-TPR tests were used with the addition of applying a 10 °C min−1 heating rate until 500 °C. Moreover, a commercial 1% Pt/Al2O3 sample was used as reference, which was not subjected to the preheating treatment in order to avoid Pt sintering [35]. Additionally, for the most active catalyst, a stability test consisting of two reaction cycles at 300 °C (5 h), using the gas mixture (i), was developed. Before each cycle, the catalyst was subjected to the corresponding pre-oxidation treatment. For reaction product quantification, the experimental setup described for CO-TPR tests was employed. The CO conversion was calculated using Equation (1).

3. Results and Discussion

3.1. Catalysts Characterization

3.1.1. Chemical, Morphological and Structural Properties

Table 1 features the nomenclature, the BET surface area (calculated from N2 adsorption data), the chemical composition (determined by μ-XRF), and some relevant XRD data of BxM perovskites. The weight percentages follow the expected trend according to the decrease in Ba in the molecular formula and, as is usual for solids with an almost negligible porosity, these mixed oxides present very low surface areas [36,37].
The X-ray patterns of the BxM samples shown in Figure 1 reveal the presence of the hexagonal BaMnO3 perovskite structure with a P63/mmc space group (JCPDS-ICDD 71-1594) as the main crystalline phase, and, as minor crystalline phases, two very low intensity peaks corresponding to BaMn8O16 (JCPDS-ICDD 29-188) and Ba2Mn8O16 (JCPDS-ICDD 78-962) have been detected. According to the following weight percentages obtained by XRD refinement: (i) for BM, BaMn8O16 contributes 1.6%; for B0.7M, BaMn8O16 and Ba2Mn8O16 contribute 3.6% and 3.9%, respectively. The contribution of these minor phases is very low. It should be noted that, as all the BaMnO3 diffraction peaks appear at the same 2θ value, a structural distortion due to the decrease in the barium content appears not to be taking place. Consequently, the cell parameters have not been modified (Table 1), and the lattice strain seems being alleviated with the decrease in the Ba content, especially for the intermediate compositions. However, the average crystallite size of the hexagonal perovskite phase is larger for BxM than for BM. This increase, which was also observed for La0.4Sr0.4MnxTi1−xO3 perovskites, appears to be attributed to the increase in the Mn percentage in the formulations [38].

3.1.2. Surface Composition

The XPS spectra of Ba, Mn, and O are shown in Figure 2a–c, being the corresponding XPS data summarized in Table 2.
In the Mn 2p3/2 spectra (Figure 2a), the three signals usually detected for Mn (III) and Mn (IV) species are featured, indicating the presence of these two oxidation states on the surface [39,40,41]. In these spectra, the binding energy of the deconvoluted band associated with Mn (III) is lower than that corresponding to Mn (IV), since the electronic charge density of the cation decreases as the oxidation state increases [39,40,41,42]. The values of the Mn (IV)/Mn (III) ratio (calculated based on the area of the deconvoluted bands) compiled in Table 2 indicate that, on the surface of all samples, a higher amount of Mn (III) than of Mn (IV) is present and the proportion of Mn (IV) increases as the Ba content decreases. In fact, the percentages of surface Mn (III) and Mn (IV) (Mn (III)s and Mn (IV)s shown in Table 2 and calculated based on the Mn (IV)/Mn (III) ratio and on the percentages of total Mn in the sample determined by μ-XRF, and they confirm that the amount of Mn (IV) increases as the Ba content decreases. This result clearly shows that the oxidation of Mn (III) to Mn (IV) takes place on the surface as an electronic charge compensation mechanism to counteract the decrease in the positive charge in the Ba-deficient perovskites. This effect was also observed in a series of La1−xSrxMnO3 perovskites, where the substitution of La (III) by Sr (II) caused a positive charge deficiency [43], and in a BaMn1−xCuxO3 perovskites series in a previous publication by the authors [44].
However, it is important to underline that, as in the Mn 2p3/2 spectra analyzed above, the binding energies of the Mn (III) and Mn (IV) deconvolutions are not significantly different, and so the accurate determination of the oxidation states of Mn is a difficult task. Thus, to try to ensure a correct assignment, the usefulness of the analysis of the splitting found in the Mn 3s transition has been proposed [42,45,46]. However, for the BxM samples, this analysis is hindered by the overlap of Mn 3s and Ba 4d transitions [45]. As an alternative, an Mn 3p transition also appears in the literature as an additional tool for determining the oxidation state of Mn [47], and the corresponding spectra for BxM samples are presented in Figure 2b. According to J.M. Cerrato et al. [47], a Mn 3p signal close to 49 eV is detected if Mn (III) is present as the main oxidation state on the catalyst surface. So, as observed in Figure 2b, Mn (III) appears to be confirmed as the main one for BxM samples. Moreover, the B0.7M spectrum shows a shift in the Mn 3p signal towards higher binding energies, indicating a higher amount of Mn (IV) [47] compared to other samples, and this confirms the conclusion extracted based on the values of the Mn (IV)/Mn (III) ratio calculated from the data of Mn 2p3/2 transition.
The Ba 3d5/2, presented in Figure 2c, shows two signals: (i) lattice Ba at lower binding energies and (ii) barium carbonate (formed due to the air exposition [48,49] of samples) and barium oxide at higher binding energies [50]. As could be expected, lattice barium is the main species in all BxM samples.
Finally, the deconvolution of the O 1s signal, featured in Figure 2d, reveals the presence of the different contributions usually found in perovskites, which are as follows: (i) a band located at binding energies around 529 eV, associated with the presence of lattice oxygen (OL) [51]; (ii) a band located around 531 eV, related to the existence of oxygen with low oxygen coordination that corresponds to the oxygen vacancies formed on the surface (Odef) [52]; (iii) a signal at 532 eV, which indicates the presence of adsorbed oxygen, hydroxyl and carbonate groups on the catalyst surface (Oads) [53,54,55]; and (iv) a band located at approximately 533 eV, which corresponds to chemisorbed water (H2Ochem) [56,57]. In Table 2, the XPS and nominal values of the OL/(Ba + Mn) ratio for all samples are shown. The experimental values have been obtained from the area under the deconvoluted signals and the nominal ones (between parentheses), calculated from the molecular formula. It is important to underline that an experimental ratio lower than the nominal one indicates the presence of oxygen vacancies on the surface. Thus, all samples present oxygen vacancies on the surface that increase for the Ba-deficient perovskites. This finding suggests that these oxygen vacancies are being generated as an additional charge compensation mechanism to counteract the positive charge defect caused by the decrease in the Ba content.
Thus, the analysis of the surface chemistry of the BxM perovskites reveals that two charge compensation mechanisms appear to be taking place to balance the positive charge deficiency caused by the low amounts of Ba: the oxidation of Mn (III) to Mn (IV) and the generation of oxygen vacancies.

3.1.3. Redox Properties

The reducibility and the redox properties of samples were explored by developing Temperature-Programmed Reduction with H2 (H2-TPR), Temperature-Programmed Reduction with CO (CO-TPR) and Temperature-Programmed Desorption of O2 (O2-TPD) tests, as described in the experimental section.
Regarding H2-TPR tests, the H2 consumption profiles for BxM samples, along with those corresponding to Mn2O3 and MnO2 used as references, are shown in Figure 3a. For perovskite samples, three peaks are usually identified [58,59,60,61]: (i) a low temperature peak, which appears between 400 °C and 500 °C, assigned to Mn (IV)/Mn (III) being reduced to Mn (II); (ii) a low intensity signal, at temperatures between 700 °C and 800 °C, corresponding to the reduction of oxygen species; and (iii) another very low intensity band at around 900 °C, due to the reduction of bulk Mn (III) to Mn (II). It is noteworthy that the Mn (IV) reduction to Mn (III) is also shown as a low intensity shoulder at lower temperatures than those corresponding to the main peak [62,63]. Since the intensity of this shoulder increases as the barium content decreases, it is suggested that the amount of Mn (IV) in the bulk increases with the decrease Ba percentage, as was previously concluded for the surface based on the XPS data (see Table 2). Focusing the attention on the maximum of the main reduction peak (corresponding to the reduction of Mn (IV)/Mn (III) to Mn (II)), whose temperature values are included in Figure 3a, a shift towards lower temperatures is detected as the barium content is lowered, so the samples become more easily reducible. When comparing the reduction profiles for BxM samples with the corresponding Mn2O3 and MnO2 references, it is evident that the reduction processes involving Mn (IV)/Mn (III) are favored as the Ba content decreases. This trend should be related to the improved mobility of the ions located in the network (that facilitates the redox processes), which is caused not only by the existence of cation vacancies (due to the decrease in the barium content), but also by the formation of oxygen vacancies, which occurs to ensure the electroneutrality [64].
The experimental H2 consumption, estimated using the area under the H2 consumption profiles (and CuO as reference sample), is compared in Figure 3b with the nominal hydrogen consumption calculated considering either Mn (IV) (green line) or Mn (III) (red line) as the unique manganese oxidation states. As all experimental values are closer to the nominal value corresponding to Mn (III), it is suggested that Mn (III) is the main oxidation state of manganese in the bulk, contrary to what is observed on the surface according to the XPS results. Thus, these charge compensation mechanisms are suggested to take place only on the surface of the samples. In addition, some experimental points are below the theoretical line for Mn (III), which means that the total reduction in the bulk Mn is not being achieved. In fact, only the B0.7M sample achieves the 100% of Mn reduction.
Based on reactions (2) and (3), the amount of oxygen consumed during the H2-TPR experiments was calculated and is presented as the data collected in Table 3. Using these data and the total amount of oxygen available in the perovskites (as calculated from the molecular formula), the percentage of oxygen consumed during H2-TPR tests has been estimated. According to these data, the percentage of oxygen consumed increases with the decrease in the Ba content, caused by the improved ionic mobility and the increase in the amount of Mn (IV).
CO-TPR experiments were also performed to obtain information about the interaction of CO with the samples. The profiles corresponding to the CO conversion percentage, in Temperature-Programmed conditions and inert atmosphere, are shown in Figure 4 as a function of temperature (solid line for first cycle and dotted line for second one) for all BxM perovskites. During these tests, as oxygen is not available in the reaction atmosphere, the CO will be oxidized using exclusively the oxygen evolved from perovskites. All BxM samples are able to oxidize CO even though the percentages of CO conversion are low (below 10%). Additionally, in the second CO-TPR tests, the temperature for the maximum CO conversion is shifted toward lower values with respect to the first cycle for all samples; so, the oxygen employed for the CO oxidation appears to be available at lower temperatures during the second cycle. The amount of oxygen involved in the CO-TPR test, calculated from CO conversion data using the CO oxidation reaction (CO + ½ O2 → CO2), and the corresponding percentage in regard to the total amount of oxygen in the perovskite (calculated based on the perovskite formula BaxMnO3) have been included in Table 4. Note that, as expected due to the low CO conversion values, the percentages of O2 involved during the CO-TPR tests are low (around 25%, except for B0.8M), suggesting that the catalytic performance of the BxM samples for CO oxidation is highly dependent on the presence of oxygen in the reactant mixture. During the CO-TPR test, that is in the absence of oxygen, CO molecules should be adsorbed on the surface metal active sites [65,66], as the CO adsorption on oxygen vacancies implies a CO activation that is less effective than the activation on the metal active sites because the backdonation from oxygen vacancies to antibonding CO molecular orbitals does not take place [67,68]. However, despite the availability of metal active sites for CO adsorption and activation, the low CO conversion values obtained appear to be a consequence of a low supply of oxygen from the samples. Among samples, B0.8M presents the lowest CO conversions, as this perovskite involves the lowest amount of oxygen. Note that this sample shows the highest percentage of Mn (III) on the surface (XPS) and the lowest reducibility (H2-TPR) of the BxM series.
To complete the perovskite characterization, the release of O2 from samples has been estimated by developing O2-TPD tests, and the corresponding O2 evolution profiles are shown in Figure 5. For perovskites, the following three peaks are usually found [69,70,71]: (i) a low temperature peak, between 150 °C and 350 °C, due to the desorption of oxygen adsorbed on surface vacancies (α-O2); (ii) a medium temperature peak from 350 °C to 700 °C, corresponding to the desorption of oxygen that comes from the defects that are adsorbed on the lattice (α’-O2); and (iii) a high temperature peak, over 700 °C, assigned to the release of lattice oxygen (β-O2), which depends on the reduction of Mn (IV) to Mn (III) and the presence of bulk oxygen vacancies, according to the Kröeger–Vink reaction (5) [72]. Thus, this high temperature peak informs us about lattice oxygen mobility through the perovskite network, which is related to the oxidation ability [69].
2MnxMn + OxO ⇆ 2Mn′Mn + V••O + 1/2O2
In reaction (5), MnxMn and Mn′Mn correspond to lattice Mn (IV) and Mn (III), respectively; additionally, OxO is an oxygen atom placed in an O site and V••O is an oxygen vacancy with a double deficiency of electrons. Note that, for all BxM perovskites, the O2 desorbed mainly corresponds to lattice oxygen (β-O2) even though a very small emission of both α-O2 (mainly for BM) and of α′-O2 (mainly for B0.7M) is also detected. The total amount of the oxygen evolved (data included in Figure 5) increases with the decrease in the barium percentage, suggesting that more oxygen vacancies are generated inside the structure (to neutralize the excess of negative charge due to the lack of Ba (II) cations) and that the amount of bulk Mn (IV) (whose reduction to Mn (III) promotes the oxygen emission) is higher. Thus, the mobility of oxygen through the structure is promoted for Ba-deficient perovskites, assuring a higher reducibility, as already deduced from H2-TPR results (Figure 3a). Furthermore, if the temperatures of oxygen emission are compared with those corresponding to the CO conversion during CO-TPR tests (Figure 4), it is suggested that α′-O2 species promote the oxidation of CO. So, the very low amount of α′-O2 evolved by perovskites appears to explain the low CO conversions during CO-TPR tests. Finally, it should be noted also that lower amounts of oxygen are released during O2-TPD experiments than during CO-TPR and H2-TPR tests; this is as expected, since CO and H2 are reducing agents that promote the oxygen emission [73]. Thus, the highest oxygen mobility of B0.7M would be linked to the highest proportion of Mn (IV) and to its high reducibility (see H2-TPR results).

3.2. Catalytic Activity

Figure 6 shows the CO conversion profiles obtained under Temperature-Programmed Reaction conditions (CO-TPRe) for BxM samples in the three reaction gas mixtures tested, along with the corresponding to the 1% Pt/Al2O3 sample used as reference. The corresponding T50% data (which is the temperature used to achieve a 50% CO conversion) are featured in Table 5.
These results reveal that all perovskites are active in regard to catalyzing the CO oxidation to CO2, as the reaction does not take place in the absence of a catalyst (see “Uncatalyzed” profiles). Moreover, most of the Ba-deficient perovskites overcome the catalytic performance of the BM sample, mainly due to the presence of a higher amount of oxygen vacancies on the surface (see XPS results), that act as active sites for CO oxidation, and an improved reducibility and lattice oxygen mobility (see H2-TPR and O2-TPD results).
The comparison of T50% values for the reactant atmospheres tested (Table 5) reveals that the catalytic performance of the B0.9M and B0.7M samples (especially the latter one) is not significantly affected by the increase in the O2 partial pressure; however, for the BM and B0.8M perovskites, a decrease in the T50% is observed, and so the performance of these two samples is improved by an increase in the partial pressure of oxygen. However, the increase in the CO partial pressure originates a worse catalytic performance for all samples, as all the T50%,CO values increase, with B0.7M being the least affected.
Based on the literature [38,74,75,76,77], three mechanisms can drive the CO oxidation reaction in the presence of oxygen vacancies: the Langmuir–Hinshelwood (L-H), Eley–Rideal (E-R) and Mars–van Krevelen (MvK). Briefly, in the L-H mechanism, oxygen is firstly adsorbed on the oxygen vacancies; meanwhile, CO is adsorbed on the Mn sites, and both adsorbed molecules react for CO2 production. In the E-R pathway, O2 is adsorbed on the corresponding active sites and CO reacts directly from the gas phase. Finally, in the MvK mechanism, CO is adsorbed on the Mn sites, and these adsorbed species are subsequently oxidized using the lattice oxygen, being the generated oxygen vacancies refilled by taking oxygen from the reactant atmosphere. On the other hand, as commented above, the metal species are the main adsorption sites for CO [67,68], while oxygen vacancies play an essential role in O2 adsorption. However, computational studies [78] have demonstrated that O2 adsorption and activation can also occur on the metal active sites; therefore, a competition between CO and O2 by active sites could be taking place. Thus, for BM and B0.8M, a strong competition of CO and O2 by the active sites could be suggested since these samples present the highest values of ΔT50%,CO, as the active sites for CO should be more limited than oxygen active sites. Thus, BM and B0.8M show the worst catalytic performance as they present a lower amount of oxygen vacancies and Mn (IV) on the surface and a lower reducibility and oxygen mobility. In fact, B0.8M also features the worst performance during CO-TPR tests, which appears to be due to its low oxygen mobility and reducibility. Regarding B0.9M, it overcomes B0.8M, as it presents a higher reducibility and a slightly increased oxygen mobility. Finally, B0.7M is the most active perovskite from the BxM series, as it features the highest amount of oxygen vacancies on the surface and the highest reducibility and lattice oxygen mobility. Additionally, considering that C. Drosou et al. proposed that the improved lattice oxygen mobility in La1−xSrxMnO3 perovskites (due to the lower positive charge in the A-site) favored the contribution of the MvK mechanism for CO oxidation reaction [76], it is suggested that the MvK mechanism could have a higher contribution in the global reaction pathway for the B0.7M perovskite.
Finally, a deeper study of the catalytic performance of the B0.7M sample has been carried out by developing two consecutive reactions at 300 °C (5 h) in 1% CO/1% O2/He reactant mixture (see Section 2 for more details), and the corresponding CO conversion profiles are featured in Figure 7. The CO conversion remains almost constant during the 5 h of reaction time of the two cycles, with the percentage for the first cycle being even higher than those previously measured in CO-TPRe conditions at 300 °C. However, for the second cycle, the CO conversion percentage is lower and more similar to the percentage observed in CO-TPRe conditions (Figure 6b).
In order to explain the performance of the B0.7M sample during isothermal reaction cycles at 300 °C, the spent catalyst was characterized by XPS. Figure 8 shows the XPS spectra (Mn 2p3/2, Mn 3p and O 1s transitions) of the spent and the fresh samples, being the data of Mn (IV)/Mn (III) and OL/(Ba + Mn) ratios, and of the BE for the Mn 3p transition, collected in Table 6. The analysis of these data reveals that the decrease in the CO conversion in the second cycle appears to be related to the decrease in the Mn (IV)/Mn (III) ratio; so, a lower amount of Mn (IV) would be present on the surface, which implies a lower reducibility and oxygen mobility. The shift of the Mn 3p signal towards lower binding energies also confirms that the amount of Mn (III) is higher for the spent sample than for the fresh one.

4. Conclusions

In this work, a series of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) samples were synthesized, characterized and tested for CO oxidation reaction under simulated gasoline and diesel exhaust conditions. The key conclusions drawn are the following:
  • The synthesis procedure allowed for achieving a perovskite-type structure, as confirmed by XRD data.
  • XPS data substantiated the coexistence of a Mn(IV)/Mn(III) pair on the surface of the samples and, also, the presence of surface oxygen vacancies, being particularly prominent for B0.7M.
  • The presence of oxygen vacancies in the Ba-deficient perovskites lattice allows us to increase the reducibility of samples and the oxygen mobility.
  • The higher amount of oxygen vacancies and the improved reducibility of the Ba-deficient samples boosted the catalytic activity for the CO oxidation reaction of BM, even though the 1% Pt/Al2O3 reference sample still presents the best performance.
  • B0.7M is the most active catalyst from the BxM series, as it presents the highest amount of oxygen vacancies on the surface (XPS), the highest reducibility (H2-TPR) and lattice oxygen mobility (O2-TPD and CO-TPR).

Author Contributions

The individual contributions of each author are as follows: Á.D.-V.: investigation, resources, data curation, and writing—original draft preparation; M.J.I.-G.: conceptualization, methodology, writing—review and editing, visualization, supervision, project administration, and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Spanish Government (MINCINN: PID2019-105542RB-I00 Project), the European Union (FEDER Funds) and the Generalitat Valenciana (Prometeo II 2018/076 Project). Á. Díaz-Verde thanks the University of Alicante for his predoctoral contract.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. XRD patterns of BxM samples.
Figure 1. XRD patterns of BxM samples.
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Figure 2. XPS spectra of the Mn 2p3/2 (a), Mn 3p (b), Ba 3d5/2 (c) and O 1s (d) transitions.
Figure 2. XPS spectra of the Mn 2p3/2 (a), Mn 3p (b), Ba 3d5/2 (c) and O 1s (d) transitions.
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Figure 3. H2-TPR consumption profiles for BxM samples, for Mn2O3 and MnO2 references (a), and for H2 consumption (mL H2 (g of cat)−1) (b).
Figure 3. H2-TPR consumption profiles for BxM samples, for Mn2O3 and MnO2 references (a), and for H2 consumption (mL H2 (g of cat)−1) (b).
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Figure 4. CO conversion profiles of the first (solid lines) and the second cycle (dotted lines) of CO-TPR tests for BxM samples.
Figure 4. CO conversion profiles of the first (solid lines) and the second cycle (dotted lines) of CO-TPR tests for BxM samples.
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Figure 5. O2-TPD profiles for BxM samples.
Figure 5. O2-TPD profiles for BxM samples.
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Figure 6. CO conversion profiles for BxM and 1% Pt/Al2O3 samples in the 0.1% CO/1% O2/He (a); 1% CO/1% O2/He (b); and 1% CO/10% O2/He (c) reactant mixtures.
Figure 6. CO conversion profiles for BxM and 1% Pt/Al2O3 samples in the 0.1% CO/1% O2/He (a); 1% CO/1% O2/He (b); and 1% CO/10% O2/He (c) reactant mixtures.
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Figure 7. CO conversion profiles of B0.7M at 300 °C in the 1% CO/He reactant mixture.
Figure 7. CO conversion profiles of B0.7M at 300 °C in the 1% CO/He reactant mixture.
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Figure 8. XPS spectra of the Mn 2p3/2 (a) and O 1s (b) transitions for the fresh and the spent B0.7M samples.
Figure 8. XPS spectra of the Mn 2p3/2 (a) and O 1s (b) transitions for the fresh and the spent B0.7M samples.
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Table 1. Chemical composition, specific surface area and XRD data of BxM catalysts.
Table 1. Chemical composition, specific surface area and XRD data of BxM catalysts.
Molecular FormulaNomenclatureBET Surface Area (m2 g−1)Chemical Composition (%)Cell Parameters (Å) 1Average Crystallite Size (nm)Lattice Strain
BaMnOac
BaMnO3BM96224145.74.824.83 × 10−4
Ba0.9MnO3B0.9M85827155.74.827.23 × 10−5
Ba0.8MnO3B0.8M105629155.74.826.74 × 10−5
Ba0.7MnO3B0.7M115530155.74.826.22 × 10−4
1 Considering that the relationship between the cell parameters in the hexagonal crystal system is a = b ≠ c, only a and c parameters are included in this table.
Table 2. Mn and O XPS characterization data of BxM samples.
Table 2. Mn and O XPS characterization data of BxM samples.
CatalystMn (IV)/Mn (III)Mn (III)s (%)Mn (IV)s (%)XPS OL/(Ba + Mn) (Nominal)
BM0.22041.2 (1.5)
B0.9M0.41981.0 (1.6)
B0.8M0.42181.0 (1.7)
B0.7M0.520101.1 (1.8)
Table 3. O2 consumption during H2-TPR experiments.
Table 3. O2 consumption during H2-TPR experiments.
CatalystAmount of O2 Consumed (μmol O2 (g cat)−1)Percentage of O2
Consumed (%)
BM84514
B0.9M104016
B0.8M103015
B0.7M133018
Table 4. Data for O2 involved in CO-TPR tests for BM and BxM perovskites (calculated employing the CO conversions and assuming the absence of parallel reactions).
Table 4. Data for O2 involved in CO-TPR tests for BM and BxM perovskites (calculated employing the CO conversions and assuming the absence of parallel reactions).
CatalystO2 Involved
(μmol O2 (g cat)−1) 1		Percentage of O2 Involved (%) 2CO Converted (%) 3
Cycle 1Cycle 2Cycle 1Cycle 2Cycle 1Cycle 2
BM1678 1855 27 30 8 9
B0.9M1601 1501 24 23 8 7
B0.8M1164 1029 17 15 5 5
B0.7M1763 1652 23 22 8 7
1 Calculated based on the percentages of CO conversion. 2 Calculated as: (μmol of O2 consumed/total amount of O2 in the perovskite) × 100. 3 Calculated as: (μmol of CO consumed during the experiment/total inlet amount of CO) × 100.
Table 5. T50% data for BxM and 1% Pt/Al2O3 samples.
Table 5. T50% data for BxM and 1% Pt/Al2O3 samples.
CatalystT50% (°C)ΔT50%,CO (°C) 1ΔT50%,O2 (°C) 2
0.1%CO/1%O2/He1%CO/1%O2/He1%CO/10%O2/He
BM36444636482−82
B0.9M276343352679
B0.8M313423377110−46
B0.7M24930730358−4
1% Pt/Al2O316421220448−8
1 T50% change after increasing the CO concentration from 0.1% to 1% (PO2 constant). 2 T50% change after increasing the O2 concentration from 1% to 10% (PCO constant).
Table 6. Mn and O XPS characterization data of the fresh and spent B0.7M samples.
Table 6. Mn and O XPS characterization data of the fresh and spent B0.7M samples.
SampleMn (IV)/Mn (III)XPS OL/(Ba + Mn)Mn 3p Position (eV)
Fresh B0.7M0.51.150
Spent B0.7M0.30.249
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Díaz-Verde, Á.; Illán-Gómez, M.J. Enhancing the Performance of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) Perovskites as Catalysts for CO Oxidation by Decreasing the Ba Content. Nanomaterials 2024, 14, 1334. https://doi.org/10.3390/nano14161334

AMA Style

Díaz-Verde Á, Illán-Gómez MJ. Enhancing the Performance of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) Perovskites as Catalysts for CO Oxidation by Decreasing the Ba Content. Nanomaterials. 2024; 14(16):1334. https://doi.org/10.3390/nano14161334

Chicago/Turabian Style

Díaz-Verde, Á., and M. J. Illán-Gómez. 2024. "Enhancing the Performance of BaxMnO3 (x = 1, 0.9, 0.8 and 0.7) Perovskites as Catalysts for CO Oxidation by Decreasing the Ba Content" Nanomaterials 14, no. 16: 1334. https://doi.org/10.3390/nano14161334

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